A relational expression between fluid modulus and bulk modulus of fluid-bearing rocks is derived based on the Gassmann's equation.It reflects the influences of such factors as fluid modulus and lithology on the elastic properties of fluid-bearing rocks in the process of fluid displacement.Theoretical calculation and analysis show that fluid variation has relatively small influences on rocks with relatively large rigidity(hard)

and has relatively large influences on rocks with relatively small rigidity(soft).The lower the rock porosity is

the more sensitive the bulk modulus of fluid-bearing rocks is to fluid variation

i.e.

the compressional velocity of seismic wave decreases with increasing rock porosity.When rock porosity and fluid modulus are relatively small

compressional velocity is relatively more sensitive to variation of fluid saturation.The results of fluid displacement experiments in real rock samples agree with the theoretical analysis.